A very wide range of industrial processes use liquid droplets and solid particles of irregular shapes and sizes. Grinding powders, medical inhalers, and spray painting are just a few such examples. Industrial processes including coatings produced by thermal and other sprays typically involve determinations of particle parameters—for example, particle size, shape, velocity, and position in space. The area of aircraft icing involves supercooled water droplets in the presence of ice crystals and ice particles (spherical frozen droplets). Existing techniques may not accurately and reliably measure the size of these particles. Furthermore, existing techniques may not be able to separate the liquid droplets from the ice particles and ice crystals. Under dense spray and ice crystal conditions, existing methods have difficulty with light beam obscuration which results in a loss of information.
Existing particle imaging techniques include incorporating bright-field imaging using arc flash lamps, pulsed lasers, and pulsed LEDs for illumination. These techniques typically use charge-coupled device (“CCD”) cameras or Complementary Metal Oxide Semiconductor (“CMOS”) cameras to record the shadow images of the particles. These techniques typically use collimated or nearly collimated light with diffusers to illuminate the particle field. In these techniques, however, the out of focus particles under relatively dense particle field conditions typically produce shadows that complicate the detection and measurement of the in focus particle shadow images. In addition, larger particles in the light beam path can extinguish or obscure the light beam which causes a loss of the smaller particle image at the sample volume. Such losses of images result in an unacceptable bias in the sampling statistics.
Typically, the lasers used for the particle imaging techniques are edge emitting laser diodes. The edge emitting laser diodes are generally made up of cleaved bars diced from the wafers. As a result of the high index of refraction between air and the semiconductor material, the cleaved bars facets act as mirrors. For the edge emitting laser diodes, light oscillates parallel to the active layers and escapes sideways resulting in an elliptical laser beam profile.
Unfortunately, the lasers used for the existing particle imaging techniques are at least partially coherent and monochromatic and thus produce visible diffraction rings around features and speckle. The diffraction rings and the speckle of laser radiation are detrimental to line-of-sight microscopy. Speckle and diffraction of laser radiation degrade an image quality and the background light intensity distribution that becomes noisy and non-uniform.